at least for me. I typically have > 50 tabs open at a time, and 3.5 was just getting really bogged down and had slow response times just to switch tabs. And there was some kind of bug where if I left it running for more than 12 hours or so it would start hogging 100% of one of my cpu cores, apparently not doing anything.

It had gotten so bad, that i was starting to launch chrome instead, even though it has serious usability issues with the number of tabs i tend to have open at one time.

FF3.6 has officially fixed it, for me at least.

The browser seems as responsive as Chrome was, which is to say that I can't feel any lag when using it.

The only issue I'm having now is the new tab behavior which tries to insert new tabs next to the current one. I tried getting used to that for a few days, but it's just terrible for the way i browse. An easy about:config change, though, and it's working for me again.